paramiko.ssh_exception.SSHException: Invalid key

I think this is the first slice creation attempt since the 1.7 release announcement. Our keys have not changed, but I cannot tell from the error message what the problem is:

------------------------  -----------------------------------------------------------------------------------------------------------------------------------------------
pythonhome                /usr/testbed/venv/fabric/lib/python3.9
pythonpath                /usr/testbed/venv/fabric/lib/python3.9:/usr/testbed/venv/fabric/lib/python3.9/lib-dynload
ld_library_path           /usr/testbed/venv/fabric/lib
Credential Manager        cm.fabric-testbed.net
Orchestrator              orchestrator.fabric-testbed.net
Bastion Host              bastion.fabric-testbed.net
Core API                  uis.fabric-testbed.net
Project ID                12a5a713-f87d-4ede-aa14-111f0993e7bc
Bastion Username          lbstoller_0051589302
Bastion Private Key File  /usr/testbed/fabric/bastion
Slice Private Key File    /usr/testbed/fabric/fabric
Slice Public Key File     /usr/testbed/fabric/fabric.pub
Log Level                 INFO
Log File                  /tmp/stitcher.gg51C4/fablib.83621
Version                   1.6.3
Data directory            /tmp/fablib
SSH Command Line          ssh -i {{ _self_.private_ssh_key_file }} -F /tmp/stitcher.gg51C4/work/fabric_config/ssh_config {{ _self_.username }}@{{ _self_.management_ip }}
Token File                /tmp/stitcher.gg51C4/token.json
Sites to avoid
Bastion SSH Config File   /tmp/stitcher.gg51C4/work/fabric_config/ssh_config
------------------------  -----------------------------------------------------------------------------------------------------------------------------------------------
Submitting the slice request
Traceback (most recent call last):
  File "/usr/testbed/fabric/stitch-links.py", line 135, in <module>
    slice.submit()
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/fabrictestbed_extensions/fablib/slice.py", line 2031, in submit
    self.wait_ssh(
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/fabrictestbed_extensions/fablib/slice.py", line 1570, in wait_ssh
    raise e
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/fabrictestbed_extensions/fablib/slice.py", line 1559, in wait_ssh
    self.get_fablib_manager().probe_bastion_host()
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/fabrictestbed_extensions/fablib/fablib.py", line 1426, in probe_bastion_host
    raise e
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/fabrictestbed_extensions/fablib/fablib.py", line 1407, in probe_bastion_host
    result = bastion_client.connect(
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/paramiko/client.py", line 485, in connect
    self._auth(
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/paramiko/client.py", line 818, in _auth
    raise saved_exception
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/paramiko/client.py", line 730, in _auth
    key = self._key_from_filepath(
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/paramiko/client.py", line 638, in _key_from_filepath
    key = klass.from_private_key_file(key_path, password)
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/paramiko/pkey.py", line 421, in from_private_key_file
    key = cls(filename=filename, password=password)
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/paramiko/ed25519key.py", line 65, in __init__
    signing_key = self._parse_signing_key_data(data, password)
  File "/usr/testbed/venv/fabric/lib/python3.9/site-packages/paramiko/ed25519key.py", line 114, in _parse_signing_key_data
    raise SSHException("Invalid key")
paramiko.ssh_exception.SSHException: Invalid key

Exception: Invalid key
Stitcher failure. Debug log: /tmp/stitcher.gg51C4/fablib.83621
RunStitcher error: FABRIC stitcher failure